Hemorrhagic shock induced by traumatic injury is a leading cause of mortality. The first hour following injury has been termed the “golden hour,” because there is a short interval of time during which recognition and proper management of a patient with significant, ongoing bleeding can make the difference between life and death. Significant bleeding is not always clinically evident. Many severely injured patients have intracavitary bleeding, which means that bleeding from a major organ or vessel is contained within the thorax or abdomen. There is no external evidence of bleeding and as a result, suspicion and clinical signs of bleeding must be sought by the practitioner. In the field, where imaging and laboratory tests are generally not available, a change in vital signs over time may be the only indication that a patient is bleeding. Thus, during the “golden hour” one must learn to recognize the signs and symptoms of acute blood loss, then initiate fluid resuscitation and frequently estimate the patient's fluid needs in an ongoing fashion.
The problem is that humans are unable to recognize subtle, beat-to-beat vital sign changes that are indicative of bleeding. More importantly, humans are unable to detect subtle vital sign changes that lead to and are characteristic of impending hemodynamic decompensation or cardiovascular collapse, which is heralded by hypotension with bradycardia.
Thus, there is a need for tools and techniques to enable a practitioner to recognize as quickly as possible a probability that a patient is bleeding or has experienced intravascular volume loss (e.g., due to hemodialysis or dehydration) and/or to estimate a patient's current hemodynamic reserve, track the patient's hemodynamic reserve over time, and/or predict a patient's hemodynamic reserve in the future. The term “hemodynamic reserve” is a hemodynamic parameter indicative of the proportion of fluid in the vascular system between normovolemia (full reserve) and the onset of hemodynamic decompensation (no reserve). The latter is typically heralded by hypotension with bradycardia.